2.3: Part II- Measuring Using The Metric System

A. Taking Linear Measurments With A Ruler

Linear measurements in science are in metric units.  The basic unit is the METER (m).  The rulers you will be using today are CENTIMETER (cm) rulers.  There are 100 cm in a meter.  If you look at the ruler, you will see 10 hatch marks between each centimeter marking.  Each hatch mark represents a MILLIMETER (mm).  There are 10 millimeters in a centimeter.  

Materials

• 5-6 washers of various sizes
• Centimeter ruler

Procedure

1. Obtain 5 washers from your instructor.
2. Order the washers on a piece of paper from the smallest diameter to the largest, labeling them #1-5.
3. Using a centimeter ruler, record the diameter of each washer in centimeters.  See Figure 2 for and image on how to measure the diameter.
4. Record your results in Table 1.
5. Convert all your washer diameter measurements to millimeters and meters.  RECORD in Table 1.
6. Keep your washers in order as you will be using them later.

Results

Draw the following table in your lab notebook including the title of the table.

Table 2.1: Diameter Measurements of Washers with Unit Conversions

WASHER #	DIAMETER OF WASHER (cm)	DIAMETER OF WASHER (m)	DIAMETER OF WASHER (mm)
1
2
3
4
5

B. Taking Mass Measurments With An Electronic Balance

Weight measurements in science are also in metric units.  The basic unit is the gram (g).  The electronic balances you will be using today are gram balances.  The model you will be using will accurately measure to 0.01 gram.  There are 1000 grams in a kilogram.  One of the most common units used is the milligram (mg).  There are 1000 milligrams in a gram.  If you need a VERY small amount of something, you measure it in micrograms (µg).  There are 106 µg in a gram.  Some conversions are indicated below:

1000g = 1kg

1g = 1000mg

1g = 1,000,000µg (\(10^6\) µg)

1mg= 1000µg

Materials

• 5 washers of various sizes that were previously measured.
• Gram balance

Procedure

1. Press the on button and wait for the balance to display zeros on screen.
2. If the screen doesn’t display zeros, press the “zero” or “tare” button. 
3. Once the machine displays zeros (0.00g), place your washer on the center of the platform.
4. Wait for the scale to achieve a stable reading (numbers are not fluctuating).
5. Record your mass in grams in Table 2 for each washer starting from smallest to largest. 

Results

1. Draw the following table in your lab manual including the title.
2. Record your results in grams (g) and then convert those masses to kg and mgs.

Table 2.2: Weight Measurements of Washers with Unit Conversions

WASHER #	WEIGHT OF WASHER (g)	WEIGHT OF WASHER (kg)	WEIGHT OF WASHER (mg)
1
2
3
4
5

C. Volumetric Measurements

The metric unit for volume is the LITER (L).  There are 1000 milliliters (mL) in one liter.  Another common unit in volume is the microliter (µL).  There are \(10^6\) µL in one liter and 1000 µL in one milliliter.  Some common conversions are shown below:

1L = 1000mL

1L = 1,000,000µL (106µL)

1ml= 1000µL

You will need to become familiar with the different types of instrumentation and glassware that you will be using throughout this semester.  Today, we will focus on glassware and devices that measure larger volumes of liquid.  You will also determine when a particular device is appropriate to use based on the volume that you are dispensing.  The types of measuring devices are very different if you want to measure and dispense a liter vs. a milliliter!

Graduated Cylinder

You will use this to dispense large volumes that are more than 10 mL.  You will be using various size graduated cylinders, ranging from 20mL – 2000 mL (2L), in this class (Figure 3A). 

Serological Pipet

These pipettes accurately dispense volumes of 1mL to 10mL and can be used for volumes up to 50 mL.  You will be using mostly 5mL and 10mL serological pipettes in this class. 

A.

B.

C.

Figure 2.5. Instrumentation and glassware used to measure large volumes.  Figure 2.5.A illustrates several sizes of graduated cylinders.  Figure 2.4.B illustrates various sizes of serological pipettes.  Figure 2.4.C illustrates various sizes of beakers.

Materials

• 1 - 50ml beaker
• Squirt bottle with diH20
• 1 - 50 ml graduated cylinder
• 1 - gram scale
• 1 - 5ml serological pipette
• 1 pipette pump or electronic pipet aide

Procedure

Graduated Cylinder Measurements

1. Draw a table 3 in your lab manual as shown on the following page.
2. Obtain a 50 mL beaker.  Weigh and record the weight in grams on Table 3 under “Weight of container”.  This is the container you will use to weigh your water.  It is not what you will use to measure in this experiment.
3. The theoretical amount of water you will be measuring using a graduated cylinder is 43 mL.  This has been recorded in Table 3.
4. Using a squirt bottle, squirt 43 mL of water into a graduated cylinder.  Be sure to read from the bottom of the meniscus.
5. Pour the 43 mL from the graduated cylinder into the weighed beaker.
6. Weigh the beaker with the water and record on Table 3 under “weight of container and water.”
7. Determine the weight of the water and record as “weight of water only”.  
8. Convert this weight to mL.  Water has a density of 1g/mL.  Because water has a density of 1g/mL, then the number g=mLs (50ml=50g) Record this number as “actual volume dispensed”.  
9. Determine the % error for each of your measurements as follows: \[\% \text { Error }=\left|\frac{\text { Theoretical amount of water -Actual volume dispensed }}{\text { Theoretical amount of water }}\right| \times 100 \nonumber \]
10. If you are not with in an error range of +/- 5% then try again.

Serological Pipet

1. Dry the 50ml beaker you used previously. You already weighed this container in the previous exercise.  Write the weight of the container you previously attained.
2. Obtain a 5 mL serological pipet and a pipet pump.  Put on the pipet pump.  DO NOT SHOVE THE PIPET way up into the pump.   Use the dial to draw 3.7mL of deionized water into the pipet from a 50 mL graduated cylinder.
3. Dispense the water into the weighed beaker dialing in the opposite direction.  To get the last bit out of the pipet, quickly dial one way then the other.
4. Dispense the 3.7 mL into the weighed beaker and determine its mass (g).  Record this value in your table as “weight of container and water.” 
5. Determine the weight of the water and record.  If you accurately dispensed 3.7 mL, the weight difference should be very close to 3.7g.
6. Convert the weight to mL and record.
7. Determine the % error as described in the previous section and record on Table 1-3.
8. If you are not with in an error range of +/- 5% then try again

Results

Table 2.3: Measuring Volumes of Water and Determining the Accuracy of the Measurements

Device used	Theoretical volume dispensed (mL)	Weight of container (g)	Weight of container and water (g)	Weight of water only (g)	Actual volume dispensed (mL)	% Error
50mL graduated cylinder- try #1	43
50mL graduated cylinder- try #2 (if needed)	43
5mL serological pipet- try #1	3.7
5mL serological pipet- try #2 (if needed)	3.7

D. Accuracy And Precision

When making measurements, both precision and accuracy are extremely important.  Precision of a measurement refers to the closeness of repeated measurements or the reproducibility of the measurement (how small the range is), while accuracy describes how close the measurement is to the true value or expected value. 

Precision can be affected by the measuring instrument.  If a student uses a thermometer calibrated to the nearest degree to measure the temperature of a beaker of water and a second student uses a thermometer that is calibrated to the nearest ten degrees for the same exercise, the reproducibility or precision of measurements will be greater for the student using the thermometer calibrated to the nearest degree.  The thermometer with the greatest number of calibrations (smallest increments) is the most precise, and thus the reproducibility is greater.   

Accuracy, however, is dependent upon both the precision of the measuring instrument and the technical skills of the individual taking the measurement.  You will perform an experiment to determine which instrument is most accurate and precise for measuring 10ml of liquid. Once you have obtained your group data, we will use Google Docs to create a spreadsheet with the class data.  You will use the class data set to determine which instrument was most accurate and most precise. 

To calculate the “mean” or “average”, you will add all the measurements and then divide by the number of measurements.  For example, if three groups weighed a washer and the measurements for weight were 9 grams, 11 grams, and 10 grams, to calculate the mean you would add 9+10+11 and then divide by 3 (because there were 3 measurements) and you would get a mean of 10 grams.  To calculate the range, you would take the smallest measurement and subtract that from the largest measurement.  In the last example, the range would be 11-9, which would give you a range of 2.  As a class we will fill in the following tables and you will include them in your lab notebook (making sure to give EACH table a title!!).

Materials

• 1 - 50ml beaker
• Squirt bottle with diH20
• 1 - 25 ml graduated cylinder
• 1 - gram scale
• 1 - 10ml serological pipette
• 1 pipette pump or electronic pipet aide

Procedure

1. Obtain a 50 mL beaker.  Place the beaker on the scale and hit “tare.”  This will cancel out the weight of the beaker.
2. Obtain a second 50 mL beaker and use it to measure 10 mL of deionized water. Pour the measured 10 mL of water into the 50 mL beaker on the scale.  Record the weight in grams under the number “1” in table 1-5 (which means that this is your first attempt to measure 10 mL with the beaker), which is equivalent to the mL of water (because remember that water has a density of 1 g/mL).
3. Dispose of the water in the beaker that is on the scale.  Dry the beaker between measurements.  Repeat steps 1-3 for a total of 4 trials using a beaker to measure 10 mL water.
4. Obtain a 25 mL graduated cylinder and measure 10 mL of deionized water.  Pour the measured 10 mL of water into the 50 mL beaker on the scale.  Record the weight in grams under the number “1” (which means that this is your first attempt to measure 10 mL with the graduated cylinder), which is equivalent to the mL of water (because remember that water has a density of 1 g/mL).
5. Dispose of the water in the beaker that is on the scale.  Repeat steps 4-5 for a total of 4 trials using a graduated cylinder to measure 10 mL water.
6. Obtain a 10 mL serological pipet and measure 10 mL of deionized water.  Pour the measured 10 mL of water into the 50 mL beaker on the scale.  Record the weight in grams under the number “1” (which means that this is your first attempt to measure 10 mL with the serological pipet), which is equivalent to the mL of water (because remember that water has a density of 1 g/mL).
7. Dispose of the water in the beaker that is on the scale.  Repeat steps 6-7 for a total of 4 trials using a serological pipet to measure 10 mL water.
8. When finished, dump out all the water and place your used glassware on the cart in the front of the room.
9. Determine the average and the range for your data for each measuring device and record in table 1-5.
10. Record your averages for each measuring device in Table 2.1-2.6 for the class data.
11. Record your ranges for each measuring device in Table 2.1-2.7 for the class data.

Results

Table 2.4

Measuring Device Used to Measure 10 mL	Actual Volume Dispensed for 4 trials (mL)
	1	2	3	4	Average	Range
beaker
Graduated cylinder
Serological pipet

 

Table 2.5

Measuring Device Used to Measure 10 mL	Class Average Actual Volume Dispensed for 4 trials (mL)
	A	B	C	D	Class Average
Beaker
Graduated cylinder
Serological pipet

 

Table 2.6

Measuring Device Used to Measure 10 mL	Class Range of Actual Volume Dispensed for 4 trials (mL)
	A	B	C	D	Average Class Range
Beaker
Graduated cylinder
Serological pipet

Conclusion 

Use your data and the questions below to write your conclusion in your lab notebook

1. For the volume measurements, which glassware used to measure 10 mL was the most precise?  
2. Which measuring device was the least precise?  Explain in detail why you think that particular device is the most precise.
3. For the glassware used to measure 10 mL, which instrument was the most accurate?
4. Was the instrument that was most precise the same one that is the most accurate?
5. Why or why not (in your answer be sure to include which measuring device was the most precise and which one was the most accurate)?
6. What can contribute to a larger % error when making a measurement? If you had to repeat a measurement in Table 3, be sure to indicate why you think your % error was outside of the acceptable range.